334 THE MAINTENANCE OF THE INDIVIDUAL 



other form of mechanical supporting mechanism which would permit 

 the general physiological setup as we know it in land animals today. 



Protection 



It is difficult to speak of the skeleton without associating it with the 

 idea of protection. Special devices suggestive of protection are 

 scattered throughout representatives of most of the phyla. Certain 

 types of spicules in the sponges, the calcareous exoskeleton of stony 

 corals, and the thickened horny layer of other branching colonial 

 coelenterates (hydroids) probably serve for the protection of these 

 animals. Skeletal protective devices are also quite obvious in snails, 

 starfishes, sea-urchins, arthropods, armored fishes, fossil armored 

 reptiles, and turtles. 



Movement 



Movement is one of the almost universal characteristics of animals. 

 Even in the protozoa special locomotor organs such as pseudopodia, 

 flagella, and cilia are found. The earthworm uses its setae in crawling. 



The greatest use of the skeleton for movement, however, occurs in 

 the arthropods and vertebrates, two highly specialized groups. The 

 former have well-developed exoskeletons while the latter are charac- 

 terized by an endoskeleton. This means that in the case of insects, 

 for example, the muscles are inside the skeleton while in vertebrates 

 they are outside. In both groups, however, the skeletal elements 

 articulate with one another, usually by means of curved and rounded 

 surfaces permitting free movement of one part upon the other. 



SECTION B. DEVICES FOR MOVEMENT 



The " Why " of Motion and Locomotion 



In the first place, animals must actively seek food and must be 

 constantly on the move if they are to keep from starving. In addi- 

 tion, many animals, especially the higher vertebrates, give evidence 

 of enjoying play, another type of muscular activity. This is more 

 apt to be true of the young, but is also characteristic of many 

 adults. If an organism is to survive in the struggle for existence, the 

 ability to become adapted to different environments by moving from 

 one place to another is a third essential. For example, grazing ani- 



